[Al Skierkiewicz]

1. Does the robot run OK if you lift it off the floor? If it does then weight/friction/mis-alignment of parts is the probable cause.
2. If it does run but is slow, check the condition of the speed controllers are they showing full forward or full reverse lights when pushed to maximum? If yes then any of the above could be the cause or high resistance connections. Check the crimps and tighten the screws of every electrical connection. If the speed controllers are not going to full throttle then suspect software.
3. If it doesn't run when off the floor and the speed controllers are showing max speed, then measure the voltage being fed to the motor with a voltmeter. It should read the same as the battery when at full throttle. Be sure to place the probes on the motor side of the last connection before the motor. If not, any connection between the motor and the battery could be faulty including those at the battery terminals.
4. Motors can be damaged by a lot of heat. Is this the case? If you are over working the motors and they get hot (remember the cims have no fan or anyway of circulating air) there is a possibility that the windings inside have shorted. The condition results in low power out, high current in and a burned smell when near the motor. Sorry, there is no coming back from this failure, replacement is the only answer.
If you check all the above and still can't determine the cause, take a known good motor and connect it to the speed controller and see if it will run. If that is OK then suspect a bad motor.
Let me know what you find.